The present invention relates generally to wireless communication devices and in particular to an envelope tracking amplification system of a wireless communication device.
Power amplifiers for wireless transmission applications, such as radio frequency (RF) power amplifiers, are utilized in a wide variety of communications and other electronic applications. Ideally, the input-output transfer function of a power amplifier should be linear, that is, should implement a constant gain adjustment and phase adjustment of an input signal, wherein a perfect replica of the input signal, increased in amplitude, appears at the output of the power amplifier.
In addition, for greater efficiency, various RF systems, such as cellular systems, attempt to run power amplifiers at or near their saturation levels, in which the actual output power of the amplifier is just below its maximum rated power output level. This power output level is generally related to the supply voltage (or supply power) to the power amplifier, such that a greater supply voltage will produce a correspondingly greater output power from the amplifier; for higher power input signals, a correspondingly greater actual power output is required to maintain the amplifier at or near saturation. In various prior art amplifiers, however, the supply voltage to the power amplifier is fixed. Given a typical usage situation in which actual power output from the amplifier may vary by a range of several orders of magnitude, use of a fixed supply voltage is highly inefficient, as output power is often an order of magnitude below its maximum, and the power amplifier is not maintained at or near its saturation levels.
Various techniques have evolved to vary the supply voltage to maintain the power amplifier at or near saturation. One such technique is power supply modulation (PSM) that varies, or modulates, the supply voltage to the power amplifier in order to maintain the power amplifier at or near saturation while the input signal varies over time. For PSM, the supply voltage of the amplifier tracks the input signal variations, typically utilizing a signal detector in conjunction with a tracking power supply. In the prior art, however, the various PSM techniques have generally been limited to narrowband applications, or have poor efficiency characteristics.
For example, one prior art PSM technique, known as xe2x80x9cenvelope elimination and restorationxe2x80x9d (EER), utilizes a limiter to provide an essentially constant drive level to the power amplifier to maintain the amplifier in a hard saturation state and increase efficiency. Use of the limiter, however, greatly expands the bandwidth of the RF signal input to the amplifier and requires very accurate tracking of the input signal envelope, with a power supply switching frequency approximately ten times greater than the bandwidth of the RF input signal. As these switching frequencies increase, the transistors within the tracking power supply become less efficient, resulting in excessive power losses. The resulting bandwidth expansion of the limiter also requires the bandwidth capability of the amplifier to be significantly greater than the input signal bandwidth, limiting the EER configuration to narrow bandwidth applications, such as amplitude modulation (AM) RF broadcasts.
Another prior art PSM technique, known as xe2x80x9cenvelope tracking,xe2x80x9d does not utilize the limiter of EER. Consequently, envelope tracking power amplification systems may be more suitable for higher bandwidth applications. Since envelope tracking power amplification systems modulate the supply voltage provided to the power amplifier, in order to maintain linearity the power amplifier of an envelope tracking power amplifier system must be capable of maintaining a constant gain across varying supply voltages. Operational constraints of the envelope tracking power amplification system are then defined by the supply voltages at which a specified level of gain can no longer be achieved.
Typically, envelope tracking amplification systems are tuned for optimal gain and intermodulation distortion (IM) only at a maximum tracking power supply voltage. The maximum tracking power supply voltage is a rated voltage, that is, a maximum drain (for a metal-oxide semiconductor field effect transistor (MOSFET)) or collector (for a bipolar junction transistor) supply voltage, of a power transistor included in the power amplifier, which power transistor amplifies an input signal coupled to the envelope tracking amplification system. However, tuning at a maximum tracking power supply voltage may yield a sub-optimal range of supply voltages across which a constant gain may be achieved, resulting in a limited range of linear operation of the envelope tracking amplification system.
Therefore, there is a need for a method for tuning an envelope tracking amplification system that results in a constant gain across a wider range of supply voltages than the current tuning methods, thereby producing an envelope tracking amplification system of improved linearity.